Apparatus and method for pulse compression and pulse generation

ABSTRACT

A circuit and method for compressing or generating a pulse, particularly a chirp pulse, which uses infinite impulse response filtering (3.2). The infinite impulse response filters (3.9, 3.10, 3.11, 3.12) simulate the responses of finite impulse response filters. This is achieved by pre-processing the signal by passing it through a pre-processing filter (3.3) which has zero response at the resonant frequencies of the infinite impulse response filters (3.9, 3.10, 3.11, 3.12). The pre-processing and infinite impulse response filtering may be incorporated into a matched filter for detection of a chirp waveform input signal. This can be implemented in the time domain or the frequency domain. The matched filter also can incorporate beating to the base band techniques. The matched filter can also act as a generator by applying a pulse to the input which has the unit impulse function causing a chirp signal, equivalent to the transfer function of the filter, to be produced as the output.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to pulse compression and pulse generation,in particular for compression or generation of chirp pulses.

2. Discussion of Prior Art

Pulse compression and generation are most commonly used in active sonarand radar applications. Active sonar or radar are used for the purposeof detection of objects by the emission of pulses and monitoring thereturn of the pulse reflections. It is not known at what point in timethe reflected pulse will return and therefore it is necessary to monitorthe return signal over the period of time in which the pulse is likelyto return. The return signal also contains other elements. These arecaused by background noise (marine and electrical), biological noise,mechanical noise and clutter. It is therefore necessary to identify thepulse reflection amongst these. Conventionally, this has been done byone of two methods; simple detection or matched filtering (correlation).

Correlation can be implemented in a number of ways but iscomputationally expensive operation and its achievement in real timerequires a high degree of processing power and demands state of the art,thus expensive, technology.

This produces a filter which is a matched to a particular pulse and isknown as a matched filter. A filter is said to be matched to a pulsewhen the transfer function (F[-t]) is the time reversed function of thepulse (F[t]). The output is said to be the compressed pulse. The widthof the compressed pulse is proportional to the inverse of the bandwidthof the pulse and the amplitude is proportional to the duration of thepulse. If a pulse with the unit impulse function is applied to a matchedfilter input, a pulse, which has a function equivalent to the transferfunction of the matched filter will be produced. In this way the matchedfilter can be used as a signal generator.

Matched filtering can be implemented in two ways, in the frequencydomain or in the time domain. In the frequency domain it is implementedby digitizing the signal and then using Fast Fourier transformalgorithms which, although efficient, require large amounts ofcomputation for large BT products. Often compromise methods are usedwhich give inaccurate results. In the time domain it is implementedusing time delays, finite impulse response [FIR] filters and summation.It can be implemented using analogue techniques which requires a largeamount of hardware or by digital algorithms which require large amountsof computation.

There are two major background elements in the return signal which needto be reduced. These are reverberation (a sonar term) or clutter (aradar term) and noise. The amount of clutter is reduced by enlarging thebandwidth. For a single frequency pulse, the bandwidth is proportionalto the inverse of the pulse length and therefore, by shortening thepulse length, the bandwidth can be enlarged. The amount of noise isreduced by enlarging the pulse length. These two are in conflict witheach other. One solution is to use a chirp waveform, a pulse withvarying frequency. The bandwidth of a chirp waveform is the differencebetween the maximum and minimum frequencies. Therefore a large pulse (toreduce noise) with a large bandwidth (to reduce clutter) can beproduced.

Often the techniques of signal processing are restricted by physical andfinancial constraints. These are weight, size and cost. For instance,the signal processing unit may be situated in a torpedo. Size and weightare therefore critical and must be kept to a minimum. Also, for thisparticular example, the signal processing unit is used only once beforebeing destroyed and therefore, costs to produce it must also be kept toa minimum. The processing also has to be carried out in real time, whichrequires greater and faster computation, and therefore more hardware.

This invention aims to provide a means by which signals can be processedusing less hardware and computation thus minimising weight, size andcost.

SUMMARY OF THE INVENTION

According to one aspect the present invention there is provided:

an electronic circuit for pulse compression or pulse generationcomprising a filter which has a pre-selected transfer function, and usesinfinite impulse response filtering to simulate the response of finiteimpulse response filtering characterised in that the pre-selectedtransfer function is a chirp waveform having discrete segments, eachsegment being of a single frequency and of integer number of wavelengthsof that frequency in duration, the duration of each of the segmentsadvantageously being the same.

Ideally, the frequencies of each segment are harmonics of one singlefundamental frequency.

In one arrangement the circuit comprises a filter unit having:

a) a pre-processing filter, and

b) an infinite impulse response (IIR) filter having a pre-determinedresonant frequency connected to the output of the pre-processing filter;

the arrangement being such that the pre-processing filter has a zerotransfer function at the resonant frequency of the IIR filter such thatthe filter unit simulates one segment of the pre-selected transferfunction.

In a second arrangement in the frequency domain the circuit comprises afilter unit having:

a) an Analogue to Digital Converter (A/D) circuit;

b) a Fast Fourier Transform (FFT) circuit whose input is connected tothe output of A/D circuit;

c) a set of time delays connected to the outputs of the FFT circuit toreceive the different respective frequency components from the FFTcircuit;

d) a reverse FFT circuit; connected to the outputs of the time delays;the arrangement being that the A/D circuit digitizes an electronic pulsepresent at the input thereof; the FFT circuit converts the digitizedelectronic pulse into the harmonic components of the fundamentalfrequency of the transfer function and the time delays delay eachcomponent by the equivalent period of time between the start of thetransfer function and the start of the segment with the same frequency.

In an alternative arrangement for operation in the time domain thecircuit comprises a matched filter having:

a) pre-processing filter;

b) a time delay module having a number of output taps connected to theoutput of the pre-processing filter;

c) IIR filters having pre-determined resonant frequencies connected tothe output taps of the time delay module;

d) a summer having a plurality of inputs to which respective outputs ofthe IIR filters are connected;

the arrangement being such that the pre-processing filter has a zerotransfer function at the resonant frequencies of each of the IIRfilters; each IIR filter simulates one segment of the pre-selectedtransfer function; the duration of the time delays is equal to theduration of the corresponding segments.

In the arrangements described above the IIR filter can be implementedusing resonators, for example, using of L-C tuned circuits,piezo-electric or quartz quarter wave resonators, microwave cavities, orstate variable filters.

It is also possible to employ positive feedback in the IIR filter tomodify the Q value.

In a practical arrangement the pre-processing filter used in the timedomain implementations may comprise a delay and a subtractor wherein theinput to the delay and the output from the delay are fed into thesubtractor.

The time delay modules where used in the invention may be implementedusing television delay lines, magnetostrictive delay lines, digitalshift registers or digital memory devices.

In a second aspect for application in the time domain the inventionprovides:

a method for compression or generation of an electronic pulse comprisingthe step of selecting a filter, which uses infinite impulse responsefiltering to simulate finite response filtering, having a pre-selectedtransfer function, the pre-selected transfer function being that of achirp wave form, characterised in that:

there are included the further steps of:

a) dividing the pre-selected transfer function into discrete separatefrequency segments, each segment being of a single frequency and ofinteger number of wavelengths of that frequency in duration,

b) providing pre-processing filters each having a zero response at oneof the respective discrete frequencies,

c) connecting the output signals from the pre-processing filters torespective IIR filters each selected to have a resonant frequencycorresponding to the frequency of the zero response of the connectedpre-processing filter, and

d) summing the output signals from the IIR filters.

When applied to the frequency domain the invention provides:

a method for compression or generation of an electronic pulse comprisingthe step of selecting a filter, which uses infinite impulse responsefiltering to simulate finite response filtering, having a pre-selectedtransfer function characterised in that the pre-selected transferfunction is that of a chirp waveform made up of discrete segments, eachsegment being of a single frequency and of integer number of wavelengthsof that frequency in duration, the frequencies of each segment beingharmonics of one single fundamental frequency, wherein:

there are included the further steps of:

a) digitizing an input signal using A/D converters

b) converting the digitized signal into the harmonic components of thefundamental frequency of the transfer function using a FFT circuit

c) delaying each harmonic component using a tapped time delay modules bythe equivalent period of time between the start of the transfer functionand the start of the segment with the same frequency;

d) converting the outputs of the time delay modules using a reversed FFTcircuit into a signal in the time domain.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the method and circuit will now be described byway of examples with reference to the accompanying drawings in which:

FIG. 1 This block diagram shows a pre-processing filter and a IIR filterwhich, when combined, simulate the response of a FIR filter.

FIG. 2 This diagram shows a chirp waveform made up of discrete segments,each segment being of a single frequency and of integer number ofwavelengths in length.

FIG. 3A This block diagram shows a matched filter implemented in thetime domain, using pre-processing filters, a time delay module, IIRfilters, and a summer.

FIG. 3B This diagram is the transfer function of the matched filter.

FIG. 4 This diagram shows the unit impulse function.

FIGS. 5A and 5B These diagrams show an IIR filter and its transientresponse respectively to a pulse which has the unit impulse function.

FIGS. 6A and 6B These diagrams show two examples of L-C resonators, aseries resonator and a parallel resonator.

FIG. 7 This diagram shows a state variable filter, which can act as aresonator and can have two inputs with a phase difference of 90 degrees.

FIG. 8 This diagram shows a pre-processing filter using delay andsubtraction.

FIG. 9 This block diagram shows a matched filter with the time delaymodule being utilised for both the delay for the matched filter and forthe delay in the pre-processing filter.

FIG. 10 This diagram shows a matched filter implemented in the frequencydomain.

FIG. 11 This diagram shows the incorporation of beating to the base bandtechnique in the present invention.

BRIEF DISCUSSION OF PREFERRED EMBODIMENTS

The principle of the present invention is illustrated by the blockdiagram shown in FIG. 1. A signal at the input 1.1 passes through apre-processing filter 1.3 and then through an IIR filter 1.2 to theoutput 1.4. The function of the IIR filter 1.2 is transformed into thefunction of a FIR filter by pre-processing the input signal with thepre-processing filter 1.3 which has zero response at the output 1.6 atthe resonant frequency of the IIR filter 1.2. The combination of thesesimulate the response of the FIR filter 1.5.

Use of the present invention enables either compression of a chirpwaveform pulse or generation of a chirp waveform pulse using a matchedfilter implemented in the time domain. A convenient form of chirpwaveform as shown in FIG. 2 is made up of discrete segments 2.1, 2.2,2.3, 2.4, each segment being of a single frequency and of integer numberof wavelengths of that frequency in duration. Preferably, all of thesegments are equal in length. Preferably, all the frequencies areharmonics of a single frequency. The matched filter as shown in FIG. 3Awith the transfer function shown in FIG. 3B comprises a tapped timedelay module 3.1, IIR filters 3.2, a pre-processing filter 3.3, and asummer 3.4. The chirp waveform pulse at the input 3.8 is fed via thepre-processing filter 3.3 through the tapped time delay module 3.1 tothe output taps 3.15, 3.16, 3.17, 3.18. Though the output of each tap3.15, 3.16, 3.17, 3.18, can have several IIR filters attached preferablyeach tap 3.15, 3.16, 3.17, 3.18 has only one IIR filter 3.9, 3.10, 3.11,3.12, attached as shown. The output response of the pre-processingfilters 3.3 have zeros at the resonant frequencies of the IIR filters3.2. This transforms the response of the components of the IIR filters3.2 into the response of FIR filters. The matched filter is matched tothe chirp waveform pulse by making the transfer function 3.14 of thefilter equal to the time reversed function of the chirp pulse. This canbe achieved by making the time delays 3.5, 3.6, 3.7 in the time delaymodule 3.1 equal to the time periods of the segments 1.1, 1.2, 1.3, ofthe chirp waveform pulse, excluding the last segment, and the resonantfrequencies of the IIR filters 3.9, 3.10, 3.11, 3.12 equal to thefrequencies of the segments 1.1, 1.2, 1.3, 1.4. Then, if a pulse withthe unit impulse function, as shown in FIG. 4, is applied to the input3.8 of the matched filter, a pulse is generated at the output 3.13 whichis equivalent to the desired transfer function 3.14 of the matchedfilter. The unit impulse function is a pulse which has infinitemagnitude 4.1 and zero duration 4.2 but has unit area 4.3.

An IIR filter 5.1, as shown in FIG. 5A and its transient response asshown in FIG. 5B, is a filter that, when a pulse with the unit impulsefunction, as shown in FIG. 4, is applied at the input 5.2, the transientresponse 5.4 at the output 5.3 has an envelope 5.5 which never becomeszero in a finite period of time. IIR filters can be implemented usinganalogue circuits, particularly using resonators. The resonators arearranged to have resonant frequencies equal to the respectivefrequencies of the segments, 1.1, 1.2, 1.3, 1.4, of the chirp wave formpulse. It is preferable that the resonators have a high Q value so thatthe duration of resonant response is long compared to that of theduration of the corresponding segment of the chirp waveform pulse. The Qof a circuit is said to be "Quality factor" and is a measure of thesharpness of the peak of the function of the gain versus frequency atthe resonant frequency of the circuit. Large values of Q can beproblematic since they require large driving powers to evoke asufficiently high response to be able to measure accurately. Theseresonators could be implemented as L-C tuned circuits, actively orpassively. FIGS. 6A and 6B show two known examples which do not requiredetailed description; a series resonant circuit 6.1 in FIG. 6A and aparallel resonant circuit 6.2 in FIG. 6B. Theoretically, positivefeedback can be used to modify the Q value. Also use could be made ofmechanical resonators such as piezo-electric of quartz quarter waveresonators (for high frequency R.F. applications), or microwavecavities. At low frequencies, state variable filters consisting of twointegrators, 7.1, 7.2, and an invertor, 7.3, may be used, as shown inFIG. 7. These have the advantage of having two possible inputs, 7.4 and7.5, which are 90 degrees out of phase relative to each other. Thesignal 7.6 produced at the output of the invertor 7.3, as well as beingfed back to the integrator 7.1, also serves as the output of thecircuit.

Pre-processing is performed using the pre-processing filter 3.3 which isdesigned to have zero response at the resonant frequencies of the IIRfilters 3.2. Theoretically this can be implemented using any standardfiltering techniques. Practically, the pre-processing filter 8.1 may beimplemented as shown in FIG. 8 using a time delay 8.2 appropriate to aparticular frequency and a subtractor 8.3. Each IIR filter 3.9, 3.10,3.11, 3.12 then requires one pre-processing filter 8.1. The subtractor8.3 in each pre-processing filter 8.1 subtracts a time delayed versionof the input signal from the signal at the input 8.4. The delayintroduced by the time delay 8.2 of a pre-processing filter 8.1 is equalto the period of the resonant frequency of the IIR filter. The same timedelays in the module 3.1 used in the matched filter shown in FIGS. 3Aand 3B can also be used for the time delays 8.2 in the pre-processingfilters 8.1. An example of this is illustrated in FIG. 9 where thepre-processing filter illustrated with reference to FIG. 8 isincorporated in the matched filter illustrated in FIG. 3A. The delays9.1, 9.2, 9.3, perform the same function as the time delay modules 3.5,3.6, 3.7, of the FIG. 3A matched filter and simultaneously the samefunction as the time delay 8.2 in the pre-processing filter 8.1. Thesubtractors 9.4, 9.5, 9.6, each correspond to the subtractor 8.3 in thepre-processing filter 8.1. The IIR filters 9.7, 9.8, 9.9, and the summer9.10 are equivalent to the IIR filters 3.2 and the summer 3.4illustrated in FIG. 3A. If any or all of the frequencies of the pulseare harmonics of a single frequency, then only one pre-processing filter8.1 is required to remove those frequencies, with a time delay 8.2 ofthe period of fundamental frequency.

There may be instances where there is difficulty in implementing asufficiently high Q resonator. The response of the resonator can becompensated for by modifying the weighting to the signal either at theinput 8.4 to the pre-processing filter or at either input 8.5 or 8.6 tothe subtractor 8.3 in the pre-processing filter 8.1. This will alsomodify the transmission waveform, and reduce its effective form factorand hence its ability to discriminate against noise. In this case eachresonator requires an individual pre-processing filter 8.1.

The tapped time delay module, 3.1, can be implemented as a single unitor as a combination of units. The time delay module, 3.1, may beimplemented by using the time it takes for a signal to propagate througha medium, for example, television delay lines or magnetostrictive delaylines. It can be implemented using analogue electronics, for exampleusing charge coupled devices or a series of sample and hold circuits.Preferably, the delays are be implemented digitally using shiftregisters or digital memory devices such as random access memoryintegrated circuits. If the delays are implemented digitally it isnecessary to convert the signal into a digital signal; this can beachieved using an analogue to digital converter circuit. If the outputof the delay module is required to be analogue, a digital to analogueconverter circuits can be used.

The summer, 3.4, adds together the outputs of the IIR filters 3.2 andcan be implemented either digitally or using analogue techniques.Preferably, the summation is carried out in a form consistent with thenature of the output of the IIR filters, 3.2.

All of the constituent parts of the matched filter as shown in FIG. 3A;the time delay module, 3.1, the pre-processing filter, 3.3, the IIRfilters, 3.2, and the summer, 3.4, can be implemented, in part or as awhole, using digital algorithms implemented using dedicated integratedcircuits, discrete digital integrated circuits, processor based circuitsor by any combination of these.

Another use of the present invention enables either compression of achirp waveform pulse or generator of a chirp waveform pulse using amatched filter implemented in the frequency domain, as shown in FIG. 10.For this arrangement the pulse comprises a chirp waveform as shown inFIG. 2 which is made up of discrete segments 2.1, 2.2, 2.3, 2.4, eachsegment being of a single frequency and of integer number of wholewavelengths of that frequency in duration, all the frequencies beingharmonics of one fundamental frequency. Preferably, all the segments areequal in length. Preferably, each segment 2.1, 2.2, 2.3, 2.4, has aunique frequency. The signal at the input 10.1 is digitized usinganalogue to digital circuits 10.2. A Fast Fourier transform (FFT)circuit, 10.3, is then used to generate the frequency components: thefundamental, 10.8, and the harmonics 10.5, 10.6, 10.7, of the signal fora given frequency. Preferably, this frequency is the same as thefundamental frequency of the pulse. Each of these components 10.5, 10.6,10.7, 10.8, consists of two parts, a real part and an imaginary part.Each of these components 10.5, 10.6, 10.7, 10.8, are then delayed byrespective delay modules, 10.9, 10.10, 10.11, 10.12. Preferably theduration of time of the time delay module 10.9, 10.10, 10.11, 10.12, isequal to the period of time between the start of the pulse and the startof the segment which has the same frequency as the component part. Iftwo or more of the segments of the pulse contain the same frequency,then the equivalent component is fed through a tapped time delay module,the delay for the taps being equal to the duration of time between thestart of the pulse and the start of the segments containing thatfrequency. The output of the delay modules are then converted back intoa signal using a reverse FFT circuit 10.4. The response of this circuitto the chirp input signal is then the compressed pulse at the output10.13.

These implementations of the present invention can also incorporatebeating to the base band as shown in FIG. 11. Beating to the base bandis frequency shifting a signal to a lower frequency. The result of thisis to produce a frequency shifted compressed pulse. The input signal isa chirp waveform pulse as shown in FIG. 2 which is made up of discretesegments 2.1, 2.2, 2.3, 2.4, each segment being of a single frequencyand of integer number of wavelengths of that frequency in duration. Ifthe matched filter is implemented in the frequency domain then all ofthe frequencies of each of the segments 2.1, 2.2, 2.3, 2.4, areharmonics of one single frequency. The signal at the input 11.5 ismultiplied, using a multiplier 11.7, by the output of a local oscillator11.1 which results in the sum and difference signals being produced;i.e. one at a relatively high frequency, one at a low frequency. It isthe signal with the smaller frequency that is usually sought. Thefrequency of the local oscillator 11.1 is chosen so that the resultingfrequency of the low frequency signal is the one desired. The signalfrom the output of the multiplier 11.7 is then passed through a low passfilter 11.3 to remove the higher frequency signal. Because it is notknown at what phase the signal is being multiplied with the localoscillator, a second phase quadrature channel is provided in which thesignal at the input 11.5 is multiplied, using a multiplier 11.6, by theoutput signal of the local oscillator 11.1 phase shifted by 90 degreesby a phase shifter 11.2. The output of the multiplier 11.8 is thenpassed through a low pass filter 11.4 identical to filter 11.3 used onthe other channel. The two channel output signals from the low passfilters 11.3 and 11.4 are then fed into identical matched filters aspreviously described except for the following. When implemented in thetime domain, the signals at the outputs of corresponding taps 3.15,3.16, 3.17, 3.18, are fed into the two inputs 7.4 and 7.5 of a resonatoras shown in FIG. 7. This performs the function of an IIR filter and,because the inputs are 90 degrees out of phase relative to each other, avector adder. The outputs 7.6 of the resonators are then summed in thesummer 3.4 to produce the output signal at the output. Alternatively,the signal at the output of each tap 3.15, 3.16, 3.17, 3.18, can bepassed through an individual IIR filter, the resulting signals at thecorresponding outputs then being vector added using vector adders. Theoutputs of the vector adders are then summed in the summer 3.4 toproduce the output signal at the output 3.13. When implemented in thefrequency domain, the outputs of the corresponding time delays 10.9,10.10, 10.11, 10.12 as shown in FIG. 10 are converted directly, usingreversed Fast Fourier transform circuit 10.4, into the output signal atthe output 10.13. All of the constituent parts of the beating to baseband technique can be implemented in part or as a whole using digitalalgorithms.

The present invention has applications in remote sensing and signalprocessing, where high resolution is usually limited by the processingand the transducers available. It is also particularly applicable inareas where size, weight and cost have to be kept to a minimum. It isappropriate for the use in active sonars and radars, as well as medicaland seismic sensing.

The method may also be applied to the spatial domain by, for example,multiplexing sequentially through the elements of an array to produce asteered received beam, corresponding to a particular spatial frequencydistribution across the face of an array.

We claim:
 1. An electronic circuit for pulse compression or pulsegeneration comprising a filter which has a pre-selected transferfunction and uses infinite impulse response filtering to simulate theresponse of finite impulse response filtering wherein said filtercomprises a filter unit having:a) a pre-processing filter, and b) aninfinite impulse response (IIR) filter having a pre-determined resonantfrequency connected to the output of the pre-processing filter;thearrangement being such that the pre-processing filter has a zerotransfer function at the resonant frequency of the IIR filter such thatthe filter unit simulates one segment of the pre-selected transferfunction, said pre-selected transfer function is that of a chirpwaveform, made up of discrete segments, each segment being of a singlefrequency and of integer number of wavelengths of that frequency induration.
 2. An electronic circuit for pulse compression or pulsegeneration as claimed in claim 1 characterised in that the duration ofeach of the segments is the same.
 3. An electronic circuit for pulsecompression or pulse generation as claimed in claim 1 characterised inthat the frequencies of each segment are harmonics of one signalfundamental frequency.
 4. An electronic circuit for pulse compressionand pulse generation as claimed in claim 1 characterised in that:thecircuit comprises a matched filter having: a) pre-processing filter; b)a time delay module having a number of output taps connected to theoutput of the pre-processing filter; c) IIR filters havingpre-determined resonant frequencies connected to the output taps of thetime delay module; d) a summer having a plurality of inputs to whichrespective outputs of the IIR filters are connected;the arrangementbeing such that the pre-processing filter has a zero transfer functionat the resonant frequencies of each of the IIR filters; each IIR filtersimulates one segment of the pre-selected transfer function; theduration of the time delays is equal to the duration of thecorresponding segments.
 5. An electronic circuit for pulse compressionand pulse generation as claimed in claim 4 characterised in that the IIRfilter is implemented using resonators.
 6. An electronic circuit forpulse compression and pulse generation as claimed in claim 4characterised in that the IIR filter uses positive feedback to modifythe Q value.
 7. An electronic circuit for pulse compression and pulsegeneration as claimed in claim 4 characterised in that the time delaymodule is implemented using television delay lines, magnetostrictivedelay lines, digital shift registers or digital memory devices.
 8. Anelectronic circuit for pulse compression and pulse generation as claimedin claim 1 characterised in that the pre-processing filter comprises ofa delay and a subtractor wherein the input to the delay and the outputfrom the delay are fed into the subtractor.
 9. An electronic circuit forpulse compression or pulse generation comprising a filter which has apre-selected transfer function and uses infinite impulse responsefiltering to simulate the response of finite impulse response filteringcharacterised in that the pre-selected transfer function is that of achirp waveform, made up of discrete segments, each segment being of asingle frequency and of integer number of wavelengths of that frequencyin duration, wherein the frequencies of each segment are harmonics ofone single fundamental frequency, the circuit comprises a filter unithaving:a) an Analogue to Digital Converter (A/D) circuit; b) a FastFourier Transform (FFT) circuit whose input is connected to the outputof A/D circuit; c) a set of time delays connected to the outputs of theFFT circuit to receive the different respective frequency componentsfrom the FFT circuit; and d) a reverse FFT circuit connected to theoutputs of the time delays;the arrangement being that the A/D circuitdigitized an electronic pulse present at the input thereof; the FFTcircuit converts the digitized electronic pulse into the harmoniccomponents of the fundamental frequency of the transfer function and thetime delays delay each component by the equivalent period of timebetween the start of the transfer function and the start of the segmentwith the same frequency.
 10. A method for compression or generation ofan electronic pulse comprising the step of selecting a filter, whichuses infinite impulse response filtering to simulate finite responsefiltering, having a pre-selected transfer function, the pre-selectedtransfer function being that of a chirp waveform, characterised inthat:there are included the further steps of: a) dividing thepre-selected transfer function into discrete separate frequencysegments, each segment being of a single frequency and of integer numberof wavelengths of that frequency in duration, b) providingpre-processing filter having a zero response at each of the respectivediscrete frequencies, c) connecting the output signal from thepre-processing filter to respective IIR filters, each selected to have aresonant frequency corresponding to the frequency of one of the zeroresponses of the pre-processing filter, and d) summing the outputsignals from the IIR filters.
 11. A method for compression or generationof an electronic pulse as claimed in claim 10 characterised in that saidmethod is implemented in part or as a whole using digital algorithmsimplemented using discrete digital circuits, dedicated integratedcircuits, processor based circuits or any combination of these.
 12. Amethod for compression or generation of an electronic pulse as claimedin any one of claim 10 characterised in that the method is used for beamsteering following time division multiplexing of the signals from arrayelements and utilising the method to separate the spatial frequency ofbeams from a number of directions.
 13. A method for compression orgeneration of an electronic pulse as claimed in claim 10 characterisedin that the method incorporates beating to the base band technique. 14.A method for compression or generation of an electronic pulse comprisingthe step of selecting a filter, which uses infinite impulse responsefiltering to simulate finite response filtering, having a pre-selectedtransfer function characterised in that the pre-selected transferfunction is that of a chirp waveform made up of discrete segments, eachsegment being of a single frequency and of integer number of wavelengthsof that frequency in duration, the frequencies of each segment beingharmonics of one single fundamental frequency, wherein:there areincluded the further steps of: a) digitizing an input signal using anA/D converter, b) converting the digitized signal into the harmoniccomponents of the fundamental frequency of the transfer function using aFFT circuit, c) delaying each harmonic component using a tapped timedelay modules by the equivalent period of time between the start of thetransfer function and the start of the segment with the same frequency;and d) converting the outputs of the time delay modules using a reversedFFT circuit into a signal in the time domain.
 15. A method forcompression or generation of an electronic pulse as claimed in claim 14characterised in that said method is implemented in part or as a wholeusing digital algorithms implemented using discrete digital circuits,dedicated integrated circuits, processor based circuits or anycombination of these.
 16. A method for compression or generation of anelectronic pulse as claimed in claim 14 characterised in that the methodis used for beam steering following time division multiplexing of thesignals from array elements and utilising the method to separate thespatial frequency of beams from a number of directions.
 17. A method forcompression or generation of an electronic pulse as claimed in claim 14characterised in that the method incorporates beating to the base bandtechnique.